Fix: don't use membarrier SHARED syscall command in liburcu-bp

One main user of liburcu-bp (lttng-ust) invokes synchronize_rcu()
repeatedly, without batching (does not use call_rcu).

Those delays introduced by sys_membarrier SHARED command significantly
impacts application startup time. Therefore, revert to not using the
membarrier SHARED command.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>